To account for the neo-arteriolization of the peri-portal fibrotic tissue around the eggs of Schistosoma mansoni in the livers of infected humans, parasite products were evaluated for endothelial cell growth factor activity. Very potent endothelial cell activation and proliferation were induced by soluble antigens from eggs (SEA) but not adult schistosomes, all in the absence of any immune cells or factors. SEA from S. japonicum also stimulated endothelial proliferation but to a much lesser degree than did that of S. mansoni; S. mekongi and S. hematobium SEA were ineffective in stimulating these cells to proliferate. All the angiogenic factors thus far elucidated have been derived from mammalian sources. That a parasitic invertebrate could be capable of producing a unique angiogenic substance or be able to mimic one normally produced by its host has important biological implications.